A pressure cylinder, particularly a slave cylinder for a hydraulic actuation of a clutch or a brake, is manufactured in series and marketed in the structural shape described herein. The piston is formed as a deep-drawn, cup-shaped, part and has a cylindrical jacket surface by which it is arranged in an internal cylinder of the housing so as to be axially displaceable. A seal, which is fixedly arranged, in the housing encircles the cylindrical jacket surface of the piston and accordingly seals the pressure space with the hydraulic fluid against the atmosphere. A base with the indentation and the hemispherical cup or socket serving as a frictionally locking and swivelable support for the transmission part of the tappet is formed at the piston. An edge that is formed around the indentation serves in the interior of the piston and accordingly in the pressure space of the hydraulic fluid as a support for the spring which holds the piston under pretensioning against the inner end face of the housing.
While a spring with the desired flat characteristic is easily accommodated in the relatively long cavity of the piston, air bubbles also remain in the latter and can not escape into a pressure medium line especially when deaerating or after the hydraulic system is filled because they are prevented from escaping by the long spatial shape in the piston or by the coils of the spring arranged therein. Also, when the pressure cylinder is arranged obliquely, it is to be expected that a quantity of residual air will remain in the hydraulic system and impairs the function of the hydraulic system by its compressibility.
DE 199 07 298.1 discloses a master cylinder of a hydraulic system for a clutch or a brake in a motor vehicle which has a housing having a connection piece for a hydraulic line leading to a slave cylinder and for a channel or duct leading to a reservoir for hydraulic fluid. A piston, which is arranged in the housing so as to be displaceable, forms a variable pressure space and has a control of the duct leading to the reservoir depending upon the position of the piston.
The shape of the pressure space appears to offer an ideal shape for complete deaeration when filling the hydraulic system. Contrary to the present invention which includes a spring, DE 199 07 298.1 discloses a master cylinder wherein a spring in the pressure space may be omitted.